Area Forecast Discussion
Issued by NWS Honolulu, HI

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000
FXHW60 PHFO 221400
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
400 AM HST Wed Feb 22 2017

.SYNOPSIS...
Strong and gusty trade winds will continue the next couple of days,
with a gradual decrease in wind speed expected from Friday into the
weekend. Clouds and showers will favor windward and mauka areas,
with brief showers occasionally spreading leeward with the strong
wind. An upper-level disturbance passing over the islands today will
allow some of the trade wind showers to become briefly heavy, with
strong thunderstorms possible over the leeward Big Island slopes
this afternoon.

&&

.DISCUSSION...
The pressure gradient S of strong high pressure cells centered NE
and NW of islands continues to support gusty trade winds. Showers
moving along with the trades are affecting mainly windward areas,
and radar shows some of the cells contain small cores of heavy
rainfall, with showers also seen spreading leeward on the smaller
islands. Strong thunderstorms have been developing over Hawaiian
Offshore waters to the SW of the islands overnight, in cloud plumes
where low-level convergence is maximized. Water vapor imagery
highlights a slow-moving, closed low aloft centered over the middle
part of the island chain that is responsible for bringing
significant instability to the mid-and upper-levels of the island
atmosphere.

The short-term forecast challenge is if (and where) surface-based
parcels will be able to tap into this instability, and if the summits
on the Big Island will be impacted with winter weather conditions.
The forecast anticipates thunderstorm development over the leeward
Big Island slopes and interior areas this afternoon, and over the
Offshore waters, but nowhere else. However, based on the 12Z
sounding data indicating a significant amount of CAPE above a strong
low-level inversion, and a mid-level wedge of dry air, the
thunderstorms that do develop will have the potential to be stronger
than what is normally seen in Hawaii. A Special Weather Statement
has been issued highlighting the threats associated with these
thunderstorms, primarily lightning, gusty winds and small hail. The
low aloft is expected to move E and away from the islands later
tonight and Thursday, with more stable conditions developing.

Trade winds will continue to deliver passing windward showers that
will favor nights and mornings as the trough departs, with low-level
winds backing slightly toward the NE Thursday and Friday. GFS
indicates potential for a slight increase in low-level moisture
Thursday night and Friday as moisture associated with an old front
rides in on the trades, but a shortwave mid-level ridge is expected
to limit total rainfall.

Global models are showing a change in the weather pattern late in
the weekend into next week, with a trend toward lighter winds, and
potentially wetter weather, but there is significant uncertainty due
to model differences. A surface low developing far NNW of the area
will likely lead to a reduction in the local pressure gradient after
Sunday, but the GFS develops another low to the E of the islands
that leads to very light winds. The ECMWF does not develop this
second low, and maintains a light to moderate ESE flow that becomes
quite wet due to the development of a nearby mid-level low. The GFS
keeps the bulk of the moisture E of the islands, and with the second
surface low, but does depict another trough aloft near the islands.
The current forecast is a blend of the two solutions, and indicates
a light to moderate ESE flow, with a broad brush approach to the PoP
forecast.

&&

.AVIATION...
High pressure north of the state will keep a breezy trade wind flow
in place through tonight. Clouds and showers will focus primarily
across windward and mauka areas, but some showers will also spill
over into leeward areas from time to time due to the strength of the
trades. Brief MVFR cigs/vsbys will be possible in passing SHRA
across windward areas, mainly through mid to late morning.
Additionally, an unstable airmass will likely result in thunderstorm
development with MVFR/IFR vsbys and the potential for gusty winds
and small hail on the Big Island this afternoon. Otherwise
predominantly VFR conditions are expected.

AIRMET Sierra is in effect for mountain obscuration across windward
sections of the Big Island, Maui and Kauai. The AIRMET may need to
be expanded to include windward portions of Oahu and Molokai later
this morning. Conditions are expected to improve in all areas by mid
to late morning, but an AIRMET for mountain obscuration may again be
needed across portions of the Big Island this afternoon as
thunderstorms develop.

AIRMET Tango remains in effect for low level turbulence over and
downwind of the terrain across all islands. Conditions are expected
to continue through the day.

&&

.MARINE...
Gales (around 34 kt) are occurring across the Pailolo and Alenuihaha
Channels, which were reflected in the overnight ASCAT pass. Strong
trades were shown elsewhere across the Hawaiian waters. Although
slightly underestimating the gales, the latest model guidance
supports this trend continuing through the day today before slightly
easing Thursday through the weekend as low pressure develops far
north-northwest of the state. As a result, the current gale warning
for the channels will remain in place through early this evening.
Elsewhere, the small craft advisory has been extended through
Thursday due to a combination of strong winds and seas up to 10 ft.

A new long-period west-northwest (310 deg) swell associated with a
recent hurricane-force low that developed over the northwest Pacific
this past weekend will gradually fill in later today, peak late
tonight into Thursday, then gradually ease into Friday. Observations
at the offshore buoys 51101/51001 northwest of Kauai are running
lower than the latest wave model predictions, which indicates this
source will show up later than anticipated across the local waters.
As a result, the morning forecast will reflect this and depict a
gradual rise beginning early this evening. If this source comes in
slightly larger than anticipated, surf will near advisory levels
along exposed north and west facing shores tonight into Thursday.

Another, slightly larger, west-northwest (310 deg) swell associated
with a large area of gales south of a developing hurricane force low
near the western end of the Aleutian Islands will fill in Thursday
night, peak late Friday and Friday night, then gradually ease
through the weekend. This source will likely generate advisory-level
surf along exposed north and west facing shores late Friday through
Saturday. A few recent altimeter passes in this region sampled 20 to
25 ft seas across the large area of gales out of the northwest,
which appeared to be in decent agreement with the wave model output.

A small northerly swell associated with a compact gale that was
analyzed around 1200 nautical miles north of the islands this
morning will fill in Thursday night and hold into Saturday before
shifting out of the northeast. The overnight ASCAT pass clipped the
western periphery of this system and showed an area of 35 to 40 kt
northerly winds favorable for the local area. An earlier altimeter
pass over this area showed seas up to 22 ft, which lined up well
with the latest wave model guidance.

Strong trade winds locally and upstream of the state will generate
near advisory-level surf along east facing shores today. Surf will
steadily ease Thursday into the weekend as the trades slightly trend
down.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Small Craft Advisory until 6 PM HST Thursday for Kauai Northwest
Waters-Kauai Windward Waters-Kauai Leeward Waters-Kauai Channel-Oahu
Windward Waters-Oahu Leeward Waters-Kaiwi Channel-Maui County
Windward Waters-Maui County Leeward Waters-Maalaea Bay-Big Island
Windward Waters-Big Island Leeward Waters-Big Island Southeast
Waters.

Gale Warning until 6 PM HST this evening for Pailolo Channel-
Alenuihaha Channel.

&&

$$

Discussion...Birchard
Aviation...Jelsema
Marine...Gibbs



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